This invention relates to a process for the production of light olefins from an oxygenate feedstream. More specifically the invention relates to a process for the removal of impurities and recovery of heat from the exothermic process for the conversion of oxygenates to olefins.
Light olefins have traditionally been produced through the process of steam or catalytic cracking. Because of the limited availability and high cost of petroleum sources, the cost of producing light olefins from such petroleum sources has been steadily increasing. Light olefins serve as feeds for the production of numerous chemicals. As the emerging economies of the Third World strain toward growth and expansion, the demand for light olefins will increase dramatically.
The search for alternative materials for light olefin production has led to the use of oxygenates such as alcohols and, more particularly, to the use of methanol, ethanol, and higher alcohols or their derivatives. These alcohols may be produced by fermentation or from synthesis gas. Synthesis gas can be produced from natural gas, petroleum liquids, and carbonaceous materials including coal, recycled plastics, municipal wastes, or any organic material. Thus, alcohol and alcohol derivatives may provide non-petroleum based routes for the production of olefin and other related hydrocarbons.
Molecular sieves such as microporous crystalline zeolite and non-zeolitic catalysts, particularly silicoaluminophosphates (SAPO), are known to promote the conversion of oxygenates to hydrocarbon mixtures. Numerous patents describe this process for various types of these catalysts: U.S. Pat. Nos. 3,928,483; 4,025,575; 4,252,479; 4,496,786; 4,547,616; 4,677,242; 4,843,183; 4,499,314; 4,447,669; 5,095,163; 5,191,141; 5,126,308; 4,973,792; and 4,861,938.
The process may be generally conducted in the presence of one or more diluents which may be present in the oxygenate feed in an amount between about 1 and about 99 molar percent, based on the total number of moles of all feed and diluent components fed to the reaction zone (or catalyst). U.S. Pat. Nos. 4,861,938 and 4,677,242 particularly emphasize the use of a diluent combined with the feed to the reaction zone to maintain sufficient catalyst selectivity toward the production of light olefin products, particularly ethylene. The above U.S. patents are hereby incorporated by reference.
The conversion of oxygenates to olefins takes place at a relatively high temperature, generally higher than about 250xc2x0 C., preferably higher than about 300xc2x0 C. In the conversion of oxygenates to olefins, as significant amount heat is released in the highly exothermic reaction. Because the reactor effluent typically is at a higher temperature than the temperature of feedstock, many methods and schemes have been proposed to manage the heat of reaction generated from the process in order to avoid problems in the operation of the process. Such events as temperature surges and hot spots in the reactor will increase the rate of catalyst deactivation and result in the production of undesirable by products such as paraffins including: methane, ethane, and propane or other undesirable by products such as carbon monoxide or coke. Processes are sought which effectively use the heat of reaction which was transferred to the reactor effluent to avoid operating problems while reducing the overall utility consumption in the conversion of the oxygenate feedstock to produce light olefins and while minimizing the production of waste streams from the process.
In a conventional naphtha cracking process for the production of light olefins, the naphtha charge stock is passed to a cracking furnace to thermally convert larger hydrocarbons into a furnace effluent comprising smaller olefinic hydrocarbons. Typically, the furnace effluent is quenched first by indirect exchange, then by an oil quench, and lastly with a water quench to cool the furnace effluent and to separate the light olefin products from any heavy hydrocarbon or pygas (pyrolysis gasoline) phase which was found in the cracking step. In the present invention, the light olefins are produced by the catalytic conversion of an oxygenate, which also produces about one mole of water for every mole of oxygenate converted. When the oxygenate is converted in the presence of a non-zeolitic molecular sieve such as SAPO-34 or SAPO-17, there is essentially no heavy hydrocarbon phase formed. Furthermore, the present invention is carried out in a fluidized bed reactor which can result in the carryover of catalyst fines from the fluidized bed reactor in the reactor effluent stream. Therefore, quench schemes are sought which recover the heat of reaction from the reactor effluent, while minimizing the production of aqueous waste streams.
These and other disadvantages of the prior art are overcome by the present invention, and a new improved process for conversion of oxygenates to hydrocarbons is provided.
The present invention provides a process for converting an oxygenate to light olefins with improved heat recovery from reactor effluent streams and improved waste recovery which minimizes overall utility requirements. In the present process, the reactor effluent is quenched with an aqueous stream in a two-stage process to facilitate the separation of hydrocarbon gases from any entrained catalyst fines, remove water and any heavy byproducts such as C6+ hydrocarbons. In addition, the process of the present invention avoids the previously unknown problem of the build up of corrosive materials, particularly organic acids such as acetic, formic and propanoic acid in the operation of a conventional single column quench system. It was discovered that the reactor effluent can contain small amounts of acetic acid which could build up in conventional quench process schemes. According to the present invention, the reactor effluent is first passed to a first stage quench tower wherein the reactor effluent is contacted with a relatively pure aqueous stream and a neutralizing agent, introduced at the top of the quench tower, to provide a hydrocarbon vapor stream and a first stage bottoms stream or waste water stream. A portion of the waste water stream withdrawn from the bottom of the quench tower is recycled to the quench tower at a point above where the reactor effluent is introduced to the quench tower. In the process of the present invention, the waste water stream produced from the first stage quench tower is a much smaller drag stream than would be produced by a single quench tower and the waste water stream of the present invention comprises heavy organic oxygenates and byproducts, such as high molecular weight alcohols and ketones, and neutralized organic acid components, in addition to any carryover of catalyst fines. Heat integration with the reactor feedstream at the particular points in the process with the hydrocarbon vapor stream withdrawn from the quench tower and with the product water from the bottom of the water stripper provide improved overall heat recovery from the reactor and provide improved operating stability for the overall process.
In one embodiment, the present invention is a process using a two-stage quench for recovering heat and removing impurities from a reactor effluent stream withdrawn from a fluidized exothermic reaction zone. The process comprises the steps of passing a preheated feedstream comprising an oxygenate to an intercondenser to at least partially vaporize the preheated feedstream by indirect heat exchange to provide a partially vaporized feedstream. The partially vaporized feedstream is passed to a feed vaporizer to fully vaporize the partially vaporized feedstream to provide a vaporized feedstream. The vaporized feedstream is passed to a feed side of a feed superheater having a feed side and an effluent side to raise the vaporized feedstream to effective conversion conditions by indirect heat exchange with a reactor effluent stream to provide a superheated feedstream. The superheated feedstream is passed to the fluidized exothermic reaction zone and therein the superheated feedstream is contacted with a particulate catalyst at conversion conditions to at least partially convert the oxygenate to produce the reactor effluent stream comprising light olefins, impurities, water and catalyst particles. The reactor effluent stream is passed to the effluent side of the feed superheater to cool the reactor effluent stream to provide a desuperheated vapor effluent stream. The desuperheated vapor effluent stream is passed to a first stage tower of a two-stage quench zone comprising the first stage tower and a second stage tower. An overhead stream comprising the light olefins and a first stage bottoms stream comprising impurities, catalyst particles, and water are recovered from the first stage tower. A first portion of the first stage bottoms stream and a neutralization stream are returned to an upper portion of the first stage tower. At least a second portion of the first stage bottoms stream is withdrawn from the process as a drag stream. The overhead stream is passed to the intercondenser to cool the overhead stream by indirect heat exchange with the preheated feedstream to provide a cooled overhead stream. The cooled overhead stream is passed to the second stage tower to separate the light olefins and water to provide a vapor product stream comprising light olefins and a purified water stream. A first portion of the purified water stream is returned to the upper portion of the first stage tower, a second portion of the purified water stream is cooled in a product heat exchanger to provide a cooled purified water stream, and the cooled purified water stream is returned to an upper portion of the second stage tower. A third portion of the purified water stream is passed to a water stripper column to provide a water stripper overhead stream and a highly purified water stream. A feedstream is preheated in a feed preheater by indirect heat exchange with the highly purified water stream to produce the preheated feedstream.
In another embodiment, the invention is a process using a two-stage quench for recovering heat and removing impurities from a reactor effluent stream withdrawn from a fluidized exothermic reaction zone. The process comprises the steps of passing a preheated feedstream comprising an oxygenate to an intercooler to at least partially vaporize the preheated feedstream by indirect heat exchange to provide a partially vaporized feedstream. The partially vaporized feedstream is passed to a feed vaporizer to fully vaporize the partially vaporized feedstream to provide a vaporized feedstream. The vaporized feedstream is passed to a feed side of a feed superheater having a feed side and an effluent side to raise the vaporized feedstream to effective conversion conditions by indirect heat exchange with a reactor effluent stream to provide a superheated feedstream. The superheated feedstream is passed to the fluidized exothermic reaction zone and therein the superheated feedstream is contacted with a particulate catalyst at conversion conditions to at least partially convert the oxygenate to produce the reactor effluent stream comprising light olefins, impurities, water and catalyst particles. The reactor effluent stream is passed to the effluent side of the feed superheater to cool the reactor effluent stream to provide a desuperheated vapor effluent stream. The desuperheated vapor effluent stream is passed to a first tower of a two-stage quench zone comprising the first tower and a second tower and an overhead stream comprising the light olefins and a first stage bottoms stream comprising impurities, catalyst particles, and water are recovered. A first portion of the first stage bottoms stream is withdrawn from the process as a drag stream. A second portion of the first stage bottoms stream and a neutralization stream are returned to provide a first stage admixture. The first stage admixture stream is passed to the intercooler to cool the first stage admixture by indirect heat exchange with the feedstream to provide a cooled first stage admixture. The first stage admixture is returned to an upper portion of the first tower. The cooled overhead stream is passed to the second tower to separate the light olefins and water to provide a vapor product stream comprising light olefins and a purified water stream. A first portion of the purified water stream is cooled in a product heat exchanger to provide a cooled purified water stream, and the cooled purified water stream is returned to an upper portion of the second tower. A third portion of the purified water stream is passed to a water stripper column to provide a water stripper overhead stream and a highly purified water stream. A feedstream is preheated in a feed preheater by indirect heat exchange with the highly purified water stream to produce the preheated feedstream.
In a further embodiment, the present invention is a two-stage quench tower process for removing impurities from a superheated reactor effluent stream withdrawn from an oxygenate conversion complex. The process comprises the steps of passing the superheated reactor effluent stream comprising light olefin, water, and organic acids to a feed/effluent exchanger to desuperheat the superheated reactor effluent stream by indirect exchange with a vaporized feedstream to provide a desuperheated stream. The desuperheated stream is passed to a first tower of a two-stage quench zone containing a first tower and a second tower. The desuperheated stream is contacted in an upper portion of the first tower with a neutralized water stream to condense at least a portion of the water to provide a first stage bottoms stream comprising water and neutralized organic acids and a first stage overhead stream comprising light olefins and water. The first stage overhead stream is passed to the second tower and is therein contacted with a cooled purified water stream to provide a light olefin product stream and a purified water stream. A first portion of the purified water stream is cooled to provide the cooled purified water stream. A second portion of the purified water stream is passed to a water stripper column to provide a high purity water stream and a stripper overhead stream. The stripper overhead stream is admixed with the first stage overhead stream, and the first stage overhead stream is cooled prior to passing the first stage overhead stream to the second stage tower, or a second portion of the purified water stream is passed to a water stripper to provide a high purity water stream and a stripper overhead stream, a portion of the first stage bottoms stream is cooled and admixed with a neutralization stream and a third portion of the purified water stream to provide the neutralized water stream. A fourth portion of the purified water stream is returned to the first tower.